STUDY ON ERECTION CONTROL SCHEME FOR LONG SPAN STEEL ARCH BRIDGE AND ITS APPLICATION ON CHAOTIANMEN YANGTZE RIVER BRIDGES

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1 STUDY ON ERECTION CONTROL SCHEME FOR LONG SPAN STEEL ARCH BRIDGE AND ITS APPLICATION ON CHAOTIANMEN YANGTZE RIVER BRIDGES Zhongfu XIANG, Xuesong ZHANG, Baisong DU, Jian MAO & Hongyue YU School of Civil Engineering & Architecture Chongqing Jiaotong Univ., Chongqing 40074, China Key words: Long-span, steel through arch, structural behaviors, erection control Abstract: The Chongqing Chaotianmen Yangtze River Bridge is a three-span steel through arch bridge currently under construction. By the time of its completion at the end of 2008, it will become the longest steel arch bridge in the world with a record span of 552 m. This paper describes some of its design characteristics and challenges in the construction control. Time dependent analyses with 3D models were carried out to study the structure behavior at each erection stage and to identify potential problems as well as applicable solutions during erection. Based on the results from analyses, a reasonable and practical scheme of erection stages and controlling procedures was proposed for structure to safely and accurately achieve its final design configuration at closure

2 1. INTRODUCTION Technology of designing and building a steel arch bridge has been significantly advanced in the nineteen century. It took about 42 years for the span length to increase from m in 1874 (St. Louis Bridge) to 297 m in 1916 (Hell Gate Bridge), an increase of 90% in span length. The construction of Bayonne Bridge in 1931 further extended span length to 504 m, representing an increase of 70% in 15 years. In 1977, New River Gorge Bridge was built with a record span of m. However, it is only a meager 3% increase in span length comparing with that of Bayonne Bridge. No new steel arch bridges built after was able to break this span record for more than twenty-five year until LuPu Bridge in Shanghai was completed in 2003 with a new record span of 550 m as shown in Table 1. No. Bridge name Location Year Completed Main span (m) 1 New River Gorge Bridge America Bayonne Bridge America Sydney Harbor Bridge Australia Fermont Bridge America Port Mann Bridge Canada Sint Michielsbrug Bridge Panama Livio Lotte Bridge Canada Runcorn Bridge England Zdakov Bridge Czech Birchenough Bridge Zimbabwe Roosevelt Lack Bridge America Table 1: Abroad list of long-span steel tied-arch bridges 2. CHONGQING CHAOTIANMEN YANGTZE RIVER BRIDGE The Chongqing Chaotianmen Yangtze River Bridge is a steel through arch bridge with double decks. It consists of a three continuous span with span layout of 190 m m m (see Figure 1). It is also a signature bridge for the city of Chongqing, China. The upper deck provides two-way three-lane roadway and pedestrian sidewalk in each direction with total deck width of 36.5 m. The lower deck carries two-way light-rail traffic at center portion with 7 m deck width on each side reserved for future usage. The main span of 552 m has set a new span record for the longest arch bridge in the world. The bridge started construction at the end of 2005 and is expected to complete at the end of The main bridge of the Chongqing Chaotianmen Yangtze River Bridge is a three-span continuous structural system with fixed hinge support on the north main pier and roller supports on other piers. Movement in the transverse direction is fixed on the main piers with a small gap between upper and lower bearing plate for the required temperature movement. Transverse movements on both end piers are free; however, shear blocks are used to limit their transverse movements

3 Figure 1: Rendering of Chongqing Chaotianmen Yangtze River Bridge Figure 2: Structural system of the bridge 3. DESIGN CHARACTERISTICS OF THE BRIDGE 3.1 Overview and structural characteristics of the bridge The bridge is a three-span (190 m +552 m +190 m) continuous steel through arch bridge with a total length of m, the truss width of 29 m, and the bridge width of 36.5 m. Two side spans are steel truss with varying height and panel spacing. The mid-span is a steel truss tied-arch bridge with flexible girder and hanger rods. The height between the arch crown and the support at springing is 142 m. The shape of arch curve is a parabola. The rise of the bridge is 128 m, and the rise-to-span ratio is 1/ A curve with a radius of 700 m is used for the top truss chord in the transition area between the middle and side span. The main arch are Pratt truss with height of 14.0 m at crown, and total height of truss at the middle support is m including arch height of m. The height of truss at the end of side span is m. Three panel spacing of 12 m, 14 m, and 16 m were used along the bridge. The layout of the side span and the middle span are 8 12 m +14 m m and 5 16 m m m m m, respectively. Two levels of steel tie girders are employed to link the bottom chord joints of the main arch span at a spacing of m. The tie girder at the upper deck level is an H-section and the tie girder at the lower deck level is a 王 -section with externally attached prestressing tendons

4 3.2 Cross-sections of main trussed members The chord members of main truss are welded built-up box section with width and depth of the section as well as plate thickness varying to accommodate member force variation. There are two section widths used: 1200 mm and 1600 mm, but the depth and plate thickness of the section varying between 1240 mm ~ 1840 mm and 24 mm ~ 50 mm, respectively. Full section splice is required for the members with the same size. Variable plates can be used for the section splice with different member size. Web members have different section type including box-section, H-section, and 王 -section, with three section width of 1,200 mm, 1,600 mm, and 1,200 ~ 1,600 mm and with depth of 1,240 mm ~ 1,440 mm for the box section, 700 mm ~ 1,100 mm for H-section, and 700 mm ~ 1,100 mm for 王 -section. The plate thickness of web members has a range of 16 mm ~ 50 mm. The tie girder at the upper-deck is an H-section with a depth of 1,500 mm and a width of 1,200 mm. The tie girder at the lower-deck is a 王 -section with a depth of 1,700 mm and a width of 1,600 mm. The maximum thickness of the truss members is 50 mm, the maximum length is 44 m, and the maximum weight is 80 t. 3.3 Main truss connection High strength bolts are used for the main truss connection at the panel joints, with some special joints fabricated in plant. 3.4 Lift points Lift points at middle and end of the main span are selected to achieve zero stress condition at the closure. 3.5 Camber The camber for the main arch span was calculated based on the dead load and one-half of live load without impact. No camber for the side spans is required. The camber at mid-span is achieved by adjusting length of hanger therefore with no camber required at member fabrication. 3.6 Bridge construction procedure Two side span trusses are erected first, following by installation of main arch trusses and hangers panel by panel symmetrically toward mid-span. After the main span arch closure, temporary tie rods are installed to engage a tied-arch loading carrying system for the final installation of double-deck system. (1)Side-span truss erection The side-span is built by cantilever method on three temporary piers. First two truss panels (#1, #2) are placed first on centering with help of tower crane; other panels are installed with erection crane. Elevation of bearings on side-span piers (P6 and P9) was initially lowered by 2.3m to provide space for the adjustments required at closure. At the erection of side-spans, the bearings on the side-span piers (P6, P9) are temporarily fixed. (2) Construction of main-span arch truss

5 The main-span arch trusses are erected by cantilever method using movable crane on standing on the completed panels. As the cantilevered arm gets longer, the overturning moment increases and the stability of structure reduces. Counterweights are thus applied on side-spans to balance the overturning moment and guying cable systems are added to improve the structure stability. At the erection of main-span arch truss, the bearings on main-span piers (P7, P8) are fixed, and longitudinally freedoms of side-span bearings (P6, P9) are released. (3) Position adjustment of main-span arch at closure The closure sequence for main arch is the following: bottom chord, top chords, diagonals, and lateral bracings. Temporary closure hinge is used to speed up bottom and top chord closure. The closure of diagonals and laterals after top and bottom chord closure requires the release of temporary bearing fixity at pier P8. Jacking forces at main-span piers and side-span piers are applied to adjust bearing position and achieve final no-stress closure. (4) Installation of the temporary tie bar After the main-span closure, temporary tie bar is installed at joint E17 of the bottom stiffening chord of mid-span and stressed. The side-span bearings are then adjusted to the final design elevation and remove the guying cable-stayed system and the counterweights in a reversed order. (5) Installation of the permanent rigid tie girders and final main-span closure Permanent tie girders for the main-span can then be installed in a sequence from top down, panel by panel using fully rotary deck derrick crane. After the closure of permanent tie girder, the temporary tie bar is removed, and the bridge deck system is placed. Analysis based on the installed bearing positions can be carried out to determine additional adjustment needed to the bearing position in vertical, longitudinal, and transverse direction to ensure the closure within required tolerance. (6)Control of the construction geometry of bridge The steel girder geometry is guaranteed primarily by the quality of fabrication and the accuracy field connection. 3.7 Bridge construction process The construction flowchart is shown in Figure CHALLENGES IN DESIGN AND CONSTRUCTION 4.1 Design challenges 1) Knowledge and experience are very limited for erection control and structural system transfer from cantilever to arch at closure. Elaborate stage modeling and analysis are required to provide control parameters for the field monitoring. 2) Variation of structure component in dimension (length, width, depth, and thickness, etc.) makes compatibility in strength, stiffness, stability and fatigue much more difficult. 3) Connections at panel joints are complicated and thus present difficulty in fabrication and field assemble. 4) The loading condition of this double-deck bridge is more complicated. Its responses to static and dynamic loads (including vehicles, train, pedestrian, temperature, wind force, pre-stress in construction, etc.) require consideration of more load case combination and local stress analysis

6 Preparation of construction Construction of dock, trestle bridge, temporary piers of side span and pre-fabricating yard Installation and degugging of 1000 t.m tower crane Construction of Pier No. P6~P9 Manufacturing, transportation and prefabricating of components No. 1 and No. 2 segments of side span Installation and degugging of girder erection crane Installation of deck derrick crane Cantilever installation of side spans, up to mid supports Installation of temporary joints for loading Construction monitoring Adjustment of side span steel girder and precise positioning of mid supports Cantilever installation of 7 segments in mid span, including tie bars and bridge decks First adjustment of channel Cantilever installation of steel arch in mid span, up to B27 Installation of cable tower, mount and prestretching of No.1 cable Second adjustment of channel Installation of southern mid span, up to B33 Loading the counterweight Mount and prestretching of No.2 cable Third adjustment of channel Installation of southern and northern mid span, up to ends of closure Adjustment of error of closure and closure of truss arch Installation and prestretching of temporary tie bars Removal of cable tower and counterweight Construction of other piers besides P6~P9 Installation of tie bars of mid span, up to mid span Installation and prestretching of external prestressed cables Adjustment of the tension force of temporary tie bars Closure of mid span tie bars Removal of temporary tie bars Installation of bridge decks Construction of bridge deck pavement and ancillary works Loading test and completion acceptance Figure 3: Construction flowchart

7 5) External lifting forces are required to achieve system transfer at zero stress at closure. 6) Combination of rigid arch and flexible tie girders in the design makes its structure and stress analysis difficult. Precision in initial position establishment and final position adjustment of middle pivot is highly required. 7) QZ145000kN cast iron spherical bearings are used for the main span support. It is one of the largest bearings in the world. 4.2 Construction challenges 1) With limited experience in erection process, providing adequate monitoring and control becomes more crucial. 2) Large size of the structural components requires heavy lifting equipment. It is difficult to install and maintain the stability of lifting equipment. 3) Special considerations such as zero stress length and selection of bolt-hole locations in fabricating panels are required to ensure that erection tolerance is not exceeded. 4) To improve stability and reduce cantilever deflection of partially erected arch, counterweights are added at side span. In addition, inclined guying cable system is also provided. 5) No initial stress at closure of main span is required. 6) To ensure zero-stress (natural) at closure, lifting forces need to be applied at the side and middle fulcrums. 7) Structure system transferring from initial cantilevered condition to completed arch system after closure, and then to tied arch system requires applying of external lifting forces for bearing adjustment. 5. NECESSITIES, OBJECTIVES, AND PRINCIPLES OF CONTROL PROCESS IN BRIDGE CONSTRUCTION The bridge erection is complicated and includes many procedures such as fabrication of steel members, side-span truss erection on centering, main-span arch erection by cantilever method, structural position adjustment by lifting, main-span arch closure, bearing plane and elevation position adjustment, and structural system transfer, etc. Factors that would potentially affect the construction quality of bridge include design parameters, fabrication accuracy, temperature variation, experience of engineers and workers, experience of management, and construction specification. A full monitoring and control program is thus necessary to prove quality control and quality assurance during erection process and to eliminate potential problems in safety and structure stability. 5.1 Objectives of construction control The objective of control is to assure the global stability of cantilevered structure at all erection stages (including pier P6 and pier P9) and stresses in all structural components (including guying cable, bowstring force and fulcrum reaction force etc.) can be maintained in a safe range. The main arch thus can closure naturally, ensuring structural stress and trail geometry match design requirements

8 5.2 Principles for construction control Under the general requirement of design and construction specification, the bridge should be built safely and smoothly with a detailed, reliable and controllable scheme, and structural adjustment plans, following these principles: pre-controlling the first, post-adjusting the second; whole stability of configuration (including P6 and P9 piers), structural stress (including tilted guy cable force, bowstring force and fulcrum reaction force etc.) and trail geometry are primarily controlled objects and whole geometry status of shaped bridge is the secondary controlled object. 6. MAIN CONTENT OF PROCESS CONTROL IN BRIDGE CONSTRUCTION All structure members are plant fabricated and assembled in the field. Cantilever erection method is used for the side spans with limited temporary falsework, while the main span is erected without temporary support in the construction. Therefore, 1) dimension and precision of the component members should be strictly controlled in process (ensured by design and manufacture process); 2) various initial status of construction (surface position of side fulcrum for truss erection, middle fulcrum position, etc.) should be controlled; 3) the construction should be monitored since high stress and insufficient stability may occur in bars due to the effect of various errors, although bar stress, local stability, steel truss stability in cantilever integration have already been taken into account in the design; 4) deflection of the steel truss in cantilever integration should be monitored so that the spatial status of the structure can be acquired in real time and compared to theoretical analysis, ensuring smooth closure and steel truss outline as required; 5) balancing loads to steel truss at the side spans and tilted buckling in cantilever integration should be controlled; 6) main trussed arch closure should be controlled (including fulcrum lifting); 7) system transfer should be controlled (including temporary bowstring setup, fulcrum leveling and positioning); 8) steel bowstring, trail string and whole structure should be monitored in construction; 9) bridge surface outline should be controlled. In detail, bridge construction process is controlled according to 123 working conditions in construction, mainly including: 6.1 Structural analysis and computation on consistency of construction and final status of bridge Consistent analysis of construction process and structural stress, deformation and stability of the shaped bridge, should be implemented strictly as required by the design calculation. By comparing key measurements to analysis results, structural analysis parameters can be unified to ensure validity for the method of controlling structure analysis, as well as the model, working conditions, structural parameters and results, establishing foundations for theoretical analysis of bridge construction control. 6.2 Plans and procedures for bridge construction, confirmation or adjustment suggestion to objective status of shaped bridge By structural analysis and computation on consistency, plans and procedures for bridge construction and objective status of shaped bridge should be discussed and confirmed with

9 the designer. They can be amended, modified, and altered according to real situation if necessary. 6.3 Process control of steel truss at side span in cantilever setup For the need of fulcrum setup at the side spans and closure of the middle span, surface position for truss erection at the side fulcrum is obtained by analysis. The safety and stability of the fulcrum is monitored and balancing loads are determined. Meanwhile, according to process simulation analysis, longitudinal position, and its pre-deviation of the temporary middle fulcrum are determined. 6.4 Forming control of steel truss at side span Control focuses on the transfer process of cantilever erection from no support to simple support. Observation is needed to the displacement of the bracket at the side spans. 6.5 Process control of fulcrum setup at middle span Control focuses on surface position, height of the fulcrum at the middle span, by adjusting with a jack and by adjusting the height of the side fulcrum, respectively. 6.6 Process control of steel truss arch in cantilever erection at middle span Included various external factors such as temperature and wind force, comparison of real and theoretical heights and stresses provides judgment to the stability and safety of the structure, possibility of problems in non-stress closure of the main span, and alert to these issues. Modification can be made accordingly. Observation should be taken to structural stress and strain of steel truss. Setup outline of trussed arch needs to be controlled. In the process of truss arch setup, measurement should be taken to the down deflection of the beam, and, side curve and vertical curve due to environmental temperature change. Structural correction coefficient (= real change/theoretical change) needs to be calculated, since it is the basic material for precise adjustment of the middle span before closure. 6.7 Control of tilted buckling and load balancing at side span Based on theoretical analysis and real measurements, position of the tilted buckling and force in its cable can be determined in the process of cantilever erection of the steel truss at the middle span. Temporary cable tower, force in the tilted cable and its adjustment are monitored to ensure anti-overturn stability. Force non-homogeneity is monitored as well as the force. Meanwhile, time and magnitude of the balancing loads need to be controlled. 6.8 Control of steel truss arch closure (the first system transfer) Based monitored temperature variation, the closure time, temporary lifting locations was estimated. According to monitoring pre-temperature change, positions of temporary side and middle fulcrums at closure are analyzed and lifting adjustments to them are presented, and sensibility of the lifting to cantilever structure is also analyzed. The process of the adjustment is monitored until the first system transfer, i.e., natural closure of the steel truss arch is accomplished. Disbenefit of various external factors should also be monitored after the transfer

10 6.9 Control of span adjustment of steel truss arch and temporary bowstring setup Determine adjusting magnitude of the surface position at the side fulcrums for span adjustment. The process of the adjustment and setup of temporary bowstring should be monitored. Also, force in the bowstring and its non-homogeneity should be monitored Process control of steel truss setup of main span trail Procedure of steel truss setup of main span trail, stresses and deformations of steel structure and longitudinal displacement of the middle fulcrum in the formed three spans are main objects to be monitored and controlled Control of steel truss for trail (the second system transfer) Steel truss for trail is permanent bowstring itself. According to temperature and other factors, when the three spans are formed, positions of the side and middle fulcrums (i.e. permanent fulcrum positions) should be analyzed and determined, magnitude of adjustment should be presented, and sensibility of the jack lifting to cantilever structure should also be analyzed. The process of the adjustment is monitored until the second system transfer, i.e., steel truss for trail closures Setup control of compliant bowstring According to design and temperature, setup time and tension of compliant bowstring (pre-stressed cable outside the body) is determined. Effect of the forming process of bowstring to the structure is monitored Monitor stressing of booms The key point is to ensure that the stressing of booms is in accordance with the design, meanwhile, to ensure its length exerts effect on bridge surface outline adjustment Control of trails and adjustment of bridge surface outline The key point is the effect of construction procedure of trails (including lower trail and automotive path) and their weights to the stress and deformation of the three spans steel bowstring arches. Control to longitudinal and transverse geometry of the bridge is also needed. 7. BRIDGE CONSTRUCTION CONTROL METHODS AND THE ESTABLISHMENT OF SYSTEMS 7.1 Construction control methods The construction of the main-span arch truss is a typical self-supported cantilever erection method. As the structure geometry is relatively difficult to adjust once it is placed and connected, construction control mostly is implemented by a combination of analytical prediction and field adjustment. Simulation of construction process and parametric studies to investigate the effects of various parameters and structure sensitivity to the variation is very crucial for an accurate control. At each erection stage, structural deflection and stresses are measured, and the modification such as lifting force required and bearing position adjustments in the next stage are predicted by analytical model

11 The main-span arch closure, which is the first system transfer, in the zero stress condition, depends on the accuracy of stress prediction from analytical model, member fabrication accuracy and temperature control, as well as bearing position adjustment. Bridge closure control of trussed-steel beam (the second system transfer) in addition to relying on the accurate theoretical analysis of conversion state, precision machining of the structural members and accurate temperature control, with the change of the vertical position in the middle support (using Jack) to achieve. 7.2 The establishment of the construction control systems The scale of the bridge is very large,the procedure is very complicated,and the technology is very difficult. It is important to establish a practical and effective control system for the bridge construction. The diagram of the system represents in Figure 4. The construction control systems of the bridge Basic Analysis of the Construction Construction of Construction of Information construction monitoring error analysis the subsystems process subsystems and forecast implementation subsystems state of of control subsystems subsystems Figure 4: Construction control system diagram 7.3 Analytical models for construction control The analytical models are established to include 123 bridge configurations and loading conditions from erection starting to completion with time dependent stage construction parameters such as lifting force, bearing jacking, and add or remove structural components. Analytical predictions of structure deflection, member stresses and structural stability condition are used in the construction control. The main stage construction configurations are shown in Figure 5(a)-(f). (a) Stage 1 (Temporary piers used for side-span erection)

12 (b) Stage 2 (Removed temporary piers 1# and 2#, and side-span erection) (c) Stage 3 (Removed all temporary piers and erection of main-span arch to panel joint #21) (d) Stage 4 (Lifting cables to adjust structural position at closure) (e) Stage 5 (Installation of tie girders with temporary tie rods installed) (f) Stage 6 (The entire bridge model) Figure 5: Main stages of construction

13 8. RESULTS OF THE IMPLEMENTATION OF CONSTRUCTION CONTROL The bridge construction is near its final stage. The construction procedures are very successful especially at some key construction stages such as side-span erection, removal of temporary piers, main arch span closure, and installation of tie girders. Because of the effective control with analytical prediction, reliable field measurement and effective adjustment, all design and specification requirements are fulfilled. In particular, the main span arch truss and rigid tie girder closure have met the design requirements for zero stress. Figure 6 shows that bridge is at its final stage of completion. Table 2 shows the field measurement data at closure comparing with allowable tolerance. REFERENCES Figure 6: The bridge near completion Direction Allowable tolerance (mm) Actual Error (mm) Longitudinal 30 9 Horizontal 20 2 Vertical 20 3 Table 2: The main arch span closure accuracy [1] Scientific Research and Design Institute of Chongqing Jiaotong Design Institute of China Railway Bridge Bureau. Design Documents of the Chongqing Chaotianmen Yangtze River Bridge. [2] Chongqing Jiaotong University & Harbour Design and Research Institute in Wuhan. Construction Controlling Scheme of the Chongqing Chaotianmen Yangtze River Bridge. [3] Department of Construction of Chongqing Chaotianmen Yangtze River Bridge. Design of Construction Organization of the Chongqing Chaotianmen Yangtze River Bridge. [4] Xiang Zhong-Fu. Control Technology of Bridge Construction [M]. Beijing: People's traffic Press,

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